The present invention relates to a color television receiver or a color monitor having a flat screen.
Color television receivers and (computer) monitors serve for converting electric signals into color images. Television receivers as well as monitors nowadays usually have an interface for various video signal formats (such as composite signals, analog or digital component signals). These signals are converted in a television receiver or monitor into analog RGB signals for controlling a cathode ray tube. The video signals supplied to a television receiver or monitor are converted in such a manner that the video signal to be displayed includes luminance and chrominance values for each individual pixel of a display screen. To display an image contained in a video signal, three electron beams (one for each base color of the additive color mixture: red, green, blue) are generated in a color image display tube of a color television receiver or monitor, said electron beams being deflected towards the corresponding pixel on the viewing screen of the color display tube in accordance with the position of the pixel information in the video signal.
In a color display tube an additive color mixture is generated by a pixel-wise superposition of three chrominance component pictures. The viewing screen of such a color display tube consists of approximately 400,000 color triads; these are phosphor dots arranged in groups of three each composed of a red shining, green shining and blue shining phosphor dot. The diameter of such a phosphor dot is approximately 0.3 mm. Each of these dots is made to shine by one of the three electron beams, which are generated by the electron beam generating system in the neck of the color display tube. The deflection unit deflects the electron beams in a manner that they successively impinge on all pixels of the viewing screen. A shadow mask is arranged in the interior of the color display tube at a spacing of approximately 15 mm to the viewing screen, said shadow mask having a hole in exact allocation to each color triad. The holes having a diameter of approximately 0.25 mm are etched into the shadow mask at regular spacings. The three electron beams meet in the respective hole of the shadow mask controlled by the common beam deflection and impinge onto the phosphor dots of the viewing screen arranged behind said hole. A large part of the electrons generated by the electron beam generation system lands on the shadow mask. This leads to a heating-up and an expansion of the shadow mask, wherein in particular the holes located on the edge of the mask may be displaced in position with respect to the phosphor dots of the viewing screen. Such a displacement usually aggravates the color purity, since each of the three electron beams is allowed to impinge only onto the phosphor dot of the viewing screen associated to it.
Besides shadow masks which are provided as masks having holes, shadow mask in the form of strip masks are also used. In these strip masks the viewing screen of a color display tube is not provided with phosphor dots but with phosphor strips. Accordingly, the shadow mask comprises strip-like openings for the individual electron beams, which are each associated to the strips on the viewing screen.
To achieve that the chrominance component pictures appear congruent, the three electron beams must impinge onto the matching phosphor dots of a color triad across the entire surface of the viewing screen. Thus, the convergence of the three electron beams is adjusted in accordance with the position of its impingement point on the viewing screen of a display tube, i.e. in accordance with the deflection (so-called dynamic convergence).
The direct spacing between two adjacent phosphor dots of the same color is called dot pitch. In conventional color display tubes, the spacing between two phosphor dots or phosphor strips of the same color increases towards the edge. The resolution of a color display tube is defined by the size of the dot pitch. A variation of the dot pitch or mask pitch is an easy means to influence the curvature of the shadow mask in a desired manner. Since, however, a dot pitch that is too large is perceived by the viewer as an interfering stripe structure, a pixel resolution minimally to be kept must be obeyed when designing a shadow mask.
During the last years, color display tubes (color cathode ray tubes) were developed with screens becoming flatter and flatter. Accordingly, the radii of curvature or mask contours of the masks (shadow masks or masks having holes) also became flat. A development of flatter and flatter masks became possible by the use of invar as mask material and by coating the masks for temperature reduction during operation. A further increase of the flatness of the masks is, however, not possible in this way. Despite all effort it was not succeeded to realize screens with shaped shadow masks, which have a fully planar screen. The reason for this is the extremely small bulging of a shadow mask that is required for such a flat screen. The main problem of an extreme flat mask is the sensitivity over mechanical strain and its strong deformation in case of local heating during normal operation.
A known solution of this problem is to be seen in so-called tension masks. By means of these tension masks it is possible to use shadow masks for absolutely flat screens. The shape of these masks is defined in that they are mechanically pre-loaded either in the vertical direction only or simultaneously in the vertical and horizontal direction. This either leads to planar or cylindrical shapes. The mask remains dimensionally stable as long as the thermal expansion of the mask during operation does not compensate for the mechanical pre-load. The disadvantage of this solution is, however, that the generation of the high mechanical pre-loads requires very massive mask frame constructions. This increases the costs and the weight of a color television receiver or monitor.
For this reason, a use of conventionally shaped masks for television receivers and monitors having a flat screen would be desirable. If such an arrangement is used, the spacing between the mask and the viewing screen extremely increases at increasing distance to the center of the screen. Accordingly, the spacing of the individual phosphor dots of a color triad on the screen increases in the direction towards the edges of a screen, so that individual phosphor dots or strips in the margins become disturbingly visible to the viewer (in case of display tubes having an image side ratio of 16:9 in particular in the lateral rim portions).
Thus, it is the object of the present invention to provide a color television receiver or a color monitor having an increased reproduction quality.
This object is solved by a television receiver or a color monitor comprising the features of claim 1.
According to the invention, a color television receiver or a color monitor includes a device, in particular an electron lens system, which may vary the mutual distance of the electron beams generated in an electron beam generating system. By reducing the mutual spacing between the generated electron beams, in accordance with the deflection of the electron beams, the spacing of the shadow mask to the viewing screen can be increased at an increasing distance to the center of the screen without having to take the known disadvantages into account.
Thereby for instance the shadow mask can be curved stronger than conventionally between the center of the screen and the rim of the screen also in case of a flat viewing screen. Thus, curved masks can be used for flatter or even absolutely planar screens, without special mask materials (e.g. invar) having to be used or without having to take a larger dot pitch, i.e. a coarser resolution in the marginal areas into consideration.
The mutual distance between the electron beams is preferably adjusted in accordance with the following formula:
s≈Tri/3*(Iasxe2x88x92q)/q
This formula defines that the mutual spacing between the electron beams in the convergence plane depends proportional on the desired size of the triad dimensions Tri and on the ratio of the spacing between the convergence plane and the mask to the spacing between the mask and the viewing screen.
An electron lens system, which may effect a variable spacing of electron beams, is in the simplest case effected by a magnetic quadrupole, which is attached in the proximity of the deflection plane. By means of such a quadrupole, the two laterial beams of the electron beams generated by the electron beam system are influenced in accordance with the deflection by the deflection field of the deflection unit. Such a double magnetic quadrupole has the advantage that the desired variation of the spacings between the electron beams can be particularly easily achieved.
A further advantageous possible realization is the use of a double controllable electrostatic deflection element e.g. in the electron beam generation system. By means of a such a deflection element the spacings can also be influenced in an aimed manner.
An advantageous connection of the above-described advantages of magnetic or electrostatic elements is a combination of an electric and a magnetic quadrupole/deflection element. Such a solution is an advantageous compromise between an inexpensive realization by means of magnetic quadrupoles and the advantageous controllability by means of electrostatic deflection elements.
A further alternative form of realization is the integration of the quadrupole functions into the deflection unit, wherein the deflection unit deviates from the ideal dynamic convergence in an aimed manner and at the same time effecting a correction of this deviation by an electrostatic or magnetic quadrupole/deflection element. In this manner an inexpensive realization can be combined with an aimed influence on the variation of the spacing of the electron beams.
Especially favorable results can be achieved when the planes of the two quadrupoles used do not fall below a certain minimum spacing.
Moreover, better results can be achieved in that the effect of both quadrupoles used is compensated for with respect to the static and dynamic convergence.